Not Applicable
1. Field of the Invention
The present invention relates to compensation of Doppler shift in a mobile communication station.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a mobile communication system, signals which are transmitted from mobile stations moving relative to a base station are subject to the well-known Doppler effect, which causes a frequency shift in the frequency received at the base station relative to that which was transmitted by the mobile station. This frequency shift is referred to herein as the Doppler shift. The Doppler shift is dependent upon the speed and direction of movement of the mobile station relative to the base station. Thus, the Doppler effect can provide an increase or a decrease in the frequency, depending on the direction of movement of the mobile station relative to the base station. The magnitude of the Doppler shift is dependent on the speed with which the mobile station is moving relative to the base station.
Existing mobile communication installations provide a form of Doppler compensation, in that the frequency detection circuitry within the base station which selects a particular signal on a particular channel can take into account a certain amount of Doppler shift in the signal.
AU 664626 relates to a method and circuit arrangement for compensating for the Doppler shift in a radio signal propagating between a base station and a mobile station when the mobile station approaches and moves past the base station. As a mobile station approaches the base station with a decreasing propagating time delay, the propagating time delay is integrated at intervals to determine how the propagating time delay varies with time. This information is used to effect a change in the radio signal frequency at a particular time to compensate for the sudden Doppler shift as the mobile station moves past the base station. In a TDMA system, the interrogation occurs at intervals equal to an integral number of time frames. In the GSM standard a time frame comprises eight consecutive time slots and a single transmission burst passes between a particular mobile station and base station in any one time frame.
Thus, in this method of Doppler compensation decisions are made in response to past and incoming signals to improve the reception at a future time, i.e. it is a reactive system. It would be desirable to implement a system which can actively compensate for Doppler shifts in an incoming signal in real time.
According to one aspect of the invention there is provided a method for compensating for Doppler shift in a signal transmitted between a mobile station and a base station in a mobile communication system, the method comprising:
determining a channel impulse response for the channel on which the signal is received;
using the channel impulse response to estimate data bits of a selected portion of the received signal;
generating a reference vector using the channel impulse response and the estimated data bits;
determining a Doppler characteristic using the selected portion of the received signal and the reference vector; and
using the Doppler characteristic to provide a Doppler shift compensation for the received signal.
According to another aspect of the invention there is provided a system for compensating for Doppler shift in a signal transmitted between a mobile station and a base station in a mobile communication system, the system comprising:
a channel impulse response determination circuit for determining a channel impulse response for the channel on which the signal is received;
an estimation circuit connected to receive the received signal on the channel impulse response and to estimate data bits of a selected portion of the received signal using the channel impulse response;
a reference generator for generating a reference vector using the channel impulse response and the estimated data bits;
circuitry for determining a Doppler characteristic using the selected portion of the received signal and the reference vector; and
a Doppler shift compensation circuit operable to use the Doppler characteristic to provide a Doppler shift compensation for the received signal.
The invention is particularly applicable in TDMA mobile communication systems where the signal comprises a transmission burst. The selected portion is located in the transmission burst close to a zero phase offset point so that the effect of the Doppler characteristic is sufficiently small that it does not corrupt the transmitted bits.
According to the GSM standard, a TDMA transmission burst comprises a training sequence which is normally used to determine the channel impulse response for the channel on which the signal is received. This is done by a convolution of the received training sequence with a stored version of the training sequence.
In existing systems, the channel impulse response is used to remove from the received signal the effects of the transmission channel on the signal, in particular multi-path and attenuation effects.
According to the GSM standard, the xe2x80x9ccleaned upxe2x80x9d and filtered signal is demodulated to remove the IQ modulation by means of which the data in the signal was transmitted. Then, the demodulated signal can be decoded to generate hard bits. This can be done by a Viterbi technique.
The Doppler shift is estimated from samples of the received signal. The estimate for Doppler shift is thus dependent on channel quality, and typically gets worse when the channel quality is poor and better as the channel quality improves. When there is no Doppler shift and the channel conditions are near the sensitivity level of a receiver, the application of a Doppler compensation algorithm degrades the performance of the receiver.
On the other hand, if there is a Doppler shift in poor channel conditions near the sensitivity level of the receiver, the receiver is not able to meet the reference sensitivity limits if a Doppler compensation algorithm is implemented.
According to one embodiment of the present invention the method for compensating for Doppler shift in a signal transmitted between a mobile station and a base station in a mobile communication system, includes:
detecting the quality of the received signal; and
implementing a Doppler shift compensation in dependence on the detected signal quality.
Thus in this embodiment Doppler shift compensation is used only or mainly in good enough channel conditions. This provides an increase in performance for the receiver. The technique is provided for ensuring that Doppler compensation does not degrade the sensitivity of the receiver.
The step of detecting signal quality can include estimating the noise energy component of the signal. This can either be used itself to generate a Doppler correction modification factor for controlling the Doppler shift compensation in dependence on the detected signal quality, or to generate a signal to noise ratio for the received signal, which would then be used to generate the Doppler correction modification factor.
The Doppler compensation can be implemented as any appropriate user defined function of signal quality. For example, it could be a linear function or a step function.
In the described embodiment, the adaptive method for Doppler correction applied to a GSM system first estimates the quality of the channel and uses the resulting modification factor to scale a calculated phase difference between a reference signal and the actual received signal.
For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which: